Electrolytic method of producing printing plates



Patented Jsn.'1e,194s' ple pictures, it is expensive. od affordsautomatic control for such processes I extent upon the most apparentlydue to the fact that the current.

ELECTROLYTIC METHOD OF PItODUCING PRINTING PLATES John J. Murray,Arlington, Mass. Application May 27', 1940, Serial No. 337,509

v 1 Claim.

This invention relates to an improved printing plate and method ofproducing the same, and more particularly to a method of increasingcontrast in connection with half-tone photographic reproductionprocesses.

In those printing processes in which the printing is accomplished-from aplane surface, and

the non-printing surfaces are removed by etching, these processes beinggenerally known as relief processes, it is quite frequently necessary toincrease the contrast between the dark and light portions of thepicture. In the present practice, this contrast is obtained by coatingby hand the darker portions of the otherwise finished plate with aresist and then further etching the exposed parts. In complex pictures,

this process is quite impracticable, and, in sim- The present methandthe plates produced by following it are not only better than thosepreviously produced, but are readily distinguishable therefrom becauseof the pyramidal shape of the dots. In following the present process,the relief printing procedure is followed up to and including theetching of the plate inthe normal manner to produce the ordinaryunimproved printing surface.

At this stage, the plate consists of a metal such as copper or zincwhich. is still covered in the unetched portions with a resist. which isgenerally termed an enamel and is the remains of the originallyphotosensitive film. The etched portions are so arranged as toleave theunetched parts in a series of dots or lines.

In accordance with this process, the half-tone plate is placed in anelectrolytic bath, for example,'one capable of depositing chromium,copper or other metal upon the surface of the exposed areas of theplate, and current is then passed to deposit metal on the plate. Theamount'of metal deposited should be varied to meet the individualrequirements of the picture; .but ordinarily a time of 8 seconds to 2minutes is employed: depending upon the strength of the sol tion. thecurrent density,and

the thickness of coating desired.

At the conclusion of the plating operation it will. be found that theplating metal has deposited to a greater extent upon the darker. or

shadowy portions of the plate, and to the least exposed areas. This isdensity is heaviest in proportionto area where the amount. of exposedmetal is-least. Thetis.

the presence of large relative amounts of enamel in the shadowy areasacts to'directthe current in that vicinitytowardthe relatively smallamount of exposed metal, thereby causing deposition of and the metallicdeposit. The highlight areas the current is relatively less concentratedin the more exposedareas. i

After this treatment, the plate is immersed in etching acid. The actionof the acid appears to take place first at the junction of theenamelhaving the least enamel and the least protective metal are thefirst to be acted upon by' the acid.

The acidbeginsits action at the edges of the enamel and works downwardlyand also toward the center of the dot, leaving a dot which is vpyramid-shaped.

' As a result of the process the contrast between the shadows and theexposed portions of the film is greatly enhanced and this enhancement iscarried, out in an'entirely automatic fashion and very inexpensively.

' trasting photographic image upon a metal plate, I

more metal in that vicinity. On the other lurid. j

After the final etching has been completed, the metal deposit may beremoved or left upon the plate, the printing being unafiected by themetal if it is desired to leave it on.

The invention is illustrated diagrammatically in the drawing in whichFigure 1 shows a crosssection of a portion of va plate It comprising a 5series of dots H covered at the top with-remnants of enamel ii; inFigure 2 the same plate is shown after the deposit of the metal 18; andin Figure 3 the plate is shown after the final etching, leaving thepyramidal shaped dots it. v j

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom. What I claim as new, and desire to secure by Letters Patent,is:

The method which comprises imposing a conetching the plate to developthe image in relief in the form of minute relief areas of metal withadjacent depressions, applying to the-relief areas an acid resistingnon-conducting resist, coating the plate over the area occupied by theimage except for that portion of the area occupied by the resist with athin electrolytic deposit of an etchable metal, such deposit beingproduced by applying to the surface of the plate an electrolytic baththrough which a single electric current is passed, whereby the .depth ofetchable metal deposited varies over the surface of the plate in inverseproportion tothe amount of etching I in any given area, subjecting theelectrop at d plate to-an etching bath, etching it therein to an extentsuflicient to remove all of the electro-.

plated metal in at-least substantial crease! the plate, and thencontinuing the etching to remove a substantial quantity of underlyingmetal. whereby the contrast of the photographic image is increased.-

- JOHN J. MURRAY.

